The present invention relates to a method and system for controlling speed of an electric motor and, more particularly, to a method and system for controlling the speed of an electric motor which has a tachometer generator, such as a rotary encoder, for measuring the rotational speed of the motor output shaft, and runs at a specified speed under the voltage supplied through switching devices.
A proportional control has been commonly practiced for controlling a rotational speed of a DC motor accurately, wherein a tachometer generator, such as a rotary encoder, is provided within the motor or on the drive shaft of the motor and the output of the tachometer is fed back so that the motor runs at a constant speed. In this method, however, a difference between a command speed and an actual speed detected by the tachometer generator, such as the rotary encoder, is fed back, and the difference cannot be reduced to zero, resulting in a permanent error in the motor speed. According to a known method for eliminating the above-mentioned speed error, a proportional control is carried out by which a feedback in proportion to the difference between the command speed and the actual motor speed is made, as well as carrying out an integration control which carries out a feedback in accordance with the result of integration of the above-mentioned difference.
Recently, DC motors are often loaded by information handling devices which render various moment of inertia and frictional torque to the motor shaft. In the conventional system, each loading device directly coupled to the motor and a control loop gain and integration parameters are adjusted so as to obtain satisfactory start-up characteristics without causing overshooting and undershooting. Further, if a power voltage to the motor varies, that is effectively identical to variation of the control loop gain. Thus, the gain needs to be readjusted, or a circuit which automatically control the gain in accordance with the power voltage needs to be added.